The invention relates to an injection molding machine for thermoplastic synthetic materials, with a substantially C-form frame which is bent open by the closing force generated by a closure device between the mold halves supported by a stationary and a movable mold platen. The invention further relates to an injection unit for an injection molding machine, with a melting cylinder and a screw disposed therein rotatably and axially displaceably by a crank drive, wherein one or several cranks of the crank drive engage a head bearing in which the screw or a shaft connected therewith is supported rotatably but fixed in the axial direction.
In hydraulic injection molding machines the movable mold platen and the screw are axially displaced via hydraulic cylinders. The closing force and the injection pressure can therefore be determined in simple manner directly from the applied hydraulic pressures.
In electric injection molding machines in which the driving of the closure device and of the injection unit is carried out by electric motors, such hydraulic pressures are not available. U.S. Pat. No. 5,536,166 therefore proposes to determine the closing force in an injection molding machine in which the movable and the stationary mold platen are connected with bars by means of a wire resistance strain gauge based on the local extension of such a bar. But since the closing force can thereby be only ascertained with gross imprecision, furthermore the published Austrian patent application A 873/97 proposes providing an hydraulic cushion in the path of the force transmission of the closure device such that the closing force can again be determined on the basis of an hydraulic pressure measurement. Such a construction, of course, entails large expenditures.
In order to be able to determine the injection pressure in an electric injection unit, it was proposed to provide a pressure sensor in the screw chamber itself. However, due to the adverse external conditions in this region, such a sensor is subject to extreme stress and, accordingly, is susceptible to errors and, due to its special construction, is expensive.
It is the task of the invention to make available devices for determining the closure force and/or the injection pressure which are, in particular, suitable for electric injection pressure which are, in particular, suitable for electric injection molding machines and permit the simple and precise determination of these parameters.
According to the invention this is achieved with an injection molding machine of the above described type thereby that for determining the closing force a distance measuring device senses the bending-open of at least one of the limbs of the C-form frame. In an injection unit of the above described type this is accomplished thereby that for determining the injection pressure a distance measuring device senses the displacement of the screw or of the shaft connected therewith.
Thus the fundamental idea of the invention comprises sensing the integral deformation of a part stressed by the particular force via a distance measuring device and to determine on that basis the corresponding force or the corresponding pressure, respectively.